Corrosive Environment Monitoring Device and Method

ABSTRACT

Provided are a device and a method for corrosive environment monitoring that have high measurement precision and enable visual observation. The corrosive environment monitoring device includes a housing, a first thin metal film, a second thin metal film, and terminals. The housing has an opening in one side thereof. The other sides of the housing than the one side are sealed to form space inside the housing. The first thin metal film extends in a direction from the bottom of the space toward the opening. The first thin metal film is resistant to corrosion by a corrosive substance and serves as a supporting member. The second thin metal film extends in the space in a direction from the bottom of the space toward the opening and is supported by the first thin metal film. The second thin metal film is susceptible to corrosion by the corrosive substance and serves as a measuring member. The terminals are disposed at both ends of the first thin metal film, where an external voltage is to be applied to the terminals. The first thin metal film includes one first thin metal film extending in a direction from the bottom of the space toward the opening. The second thin metal film is disposed on one or both sides of the one first thin metal film and extends in the space in a direction from the bottom toward the opening.

TECHNICAL FIELD

The present invention relates to a device and a method for corrosiveenvironment monitoring, each of which measures a degree of corrosion bya corrosive substance present in an indoor environment, mainly in anenvironment where electric/electronic equipment is placed.

BACKGROUND ART

An exemplary background art in the technical field is disclosed inJapanese Patent Application Laid-Open No. 2003-294606 (Patent Literature1). The environmental assessment device disclosed in this literaturebasically includes 1) an element unit that reacts with a gaseouscomponent in the environment, 2) a unit that detects the change of theelement and converts the detected data into an electric signal, and 3) aunit that stores the measured data. The device employs, as the sensingelement, thin metal films (thin metal films each having a thickness of0.1 μm and being made of one of silver, copper, iron, and stainlesssteels). The device assesses an environment where a material is placed,by measuring a time-related change of at least one property, of the thinmetal films, selected from optical reflectance, light transmittance, andelectric resistance, and detecting a gaseous component in theenvironment. When the time-related change of the electric resistance isto be measured, the thickness of a corroded layer can be calculated, andthereby the corrosion rate can be easily determined, by measuring thechange of the electric resistance caused by the change of the entirethin film (general corrosion).

A gas detecting system, which detects the change in the element andconverts the change into an electric signal, includes a gas introducingunit and a gas sensing element unit (unit corresponding to a sensor unitin the present invention). The gaseous component is fed via a suctionpump in the gas introducing unit to the gas sensing element unit. Thetechnique can provide an analyzer that is very useful for environmentalassessments for various materials, under circumstances where the globalenvironment varies.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent Application Laid-Open No.2003-294606

SUMMARY OF INVENTION Problems to be Resolved by the Invention

Electric/electronic equipment requires long-term reliability so as to bestably operated. In addition, such electric/electronic equipment employsa high-density packaging structure for higher speed and for space savingand includes a large number of electric/electronic components includingmicro wiring structures and/or thin film plating structures. To providesatisfactory reliability of the electric/electronic equipment, corrosiondamage on the electric/electronic components is to be restrained. Thisis because even slight corrosion damage on the electric/electroniccomponents causes a change in electrical properties or magneticproperties and causes failure and/or malfunction of the equipment.Demands have been made to provide a technique for simple, rapid,precise/accurate, sustainable assessment of the corrosivity of theenvironment where the electric/electronic equipment is placed. Suchtechnique is demanded so as to reflect an anticorrosion measure to thedesign and maintenance of the equipment, where the measure correspondsto the degree (magnitude) of corrosivity of the environment.

As a method for assessing the corrosivity of an environment whereelectric/electronic equipment is placed, the ISO 11844-1 standardgenerally employs a method for assessing the degrees of corrosion ofcopper, silver, aluminum, iron, and zinc, each of which has been exposedto the environment for a predetermined time. It is known that copper,silver, aluminum, iron, and zinc are corroded by any of SO₂, NO₂, andH₂S, which are corrosive substances, while the degrees of corrosion varyfrom substance to substance.

Assume that the environmental assessment method according to thebackground art and the environmental assessment device using the methodemploy a thin silver film having a thickness of 0.1 μm (100 nm) tomeasure the time-related change of electric resistance of the film. Inthis case, a time period during which the sensor can perform measurementis about one month, when used for assessing an environment with “mediumcorrosivity” prescribed in ISO 11844-1. This environment is anenvironment that may cause equipment fault by corrosion and is anenvironment in which the exposed silver film is corroded in a rate of105 to 410 nm/year. The time period during which the sensor can performmeasurement is still shorter when used for assessing an environment with“high corrosivity” or an environment with “very high corrosivity”. Theenvironment with high corrosivity is an environment where the silverfilm exposed to the environment is corroded in a rate of 410 to 1050nm/year. The environment with very high corrosivity is an environmentwhere the silver film exposed to the environment is corroded in a rateof 1050 to 2620 nm/year. Both of these environments may highly possiblycause such corrosion as to affect the reliability of the equipment andessentially require improvement in environment and/or structure. Thus,the method and device in these cases are not suitable for long-termmeasurements. Such long-term measurements become possible by increasingthe thickness of the thin film. Disadvantageously, however, with anincreasing thickness, the variation in thickness increases, and therebythe measurement precision decreases.

Assume that local corrosion of the sensor unit, such as corrosionadjacent to a portion where dust and/or salts are deposited, occurs inthe environmental assessment method according to the background art andthe environmental assessment device using the method. In this case,disadvantageously, the time period during which the sensor can performmeasurement is shortened as compared with the original time periodduring which the sensor can perform measurement.

In addition and disadvantageously, when the gas sensing element unit(sensor unit) is exposed directly to a target environment, the amount ofcorrosion varies depending on the flow rate in the target environment.

These problems or disadvantages have to be solved, when a corrosiveenvironment monitoring device measures the degree of corrosion by acorrosive substance present in an environment where electric/electronicequipment is placed.

Assume that the corrosive environment monitoring device is to be mountedadjacent to the electric/electronic equipment. In this case, thecorrosive environment monitoring device is preferably small-sized,provides accurate reflection on numerical values from the degree ofcorrosion, and has such a structure as to enable visual observation ofthe degree of corrosion.

Under these circumstances, the present invention has an object toprovide a device and a method for corrosive environment monitoring whichhave high measurement precision and by which the measured result isvisible.

Means of Solving the Problems

To achieve the object, the present invention provides, in one aspect, acorrosive environment monitoring device. The corrosive environmentmonitoring device includes a housing, a first thin metal film, a secondthin metal film, and terminals. The housing has an opening in one sidethereof. The other sides of the housing than the one side are sealed toform space inside the housing. The first thin metal film extends in adirection from the bottom of the space toward the opening. The firstthin metal film is resistant to corrosion by a corrosive substance andserves as a supporting member. The second thin metal film extends in thespace in a direction from the bottom of the space toward the opening andis supported by the first thin metal film. The second thin metal film issusceptible to corrosion by the corrosive substance and serves as ameasuring member. The terminals are disposed at both ends of the firstthin metal film, where an external voltage is to be applied to theterminals. The first thin metal film includes one first extending in adirection from the bottom of the space toward the opening. The secondthin metal film is disposed on one or both sides of the one first thinmetal film and extends in the space in a direction from the bottomtoward the opening.

The present invention provides, in another aspect, a corrosiveenvironment monitoring device. This corrosive environment monitoringdevice includes a housing, a first thin metal film, a second thin metalfilm, and measuring terminals. The housing includes space that opensonly in one side of the housing. The first is resistant to corrosion bya corrosive substance. The second is disposed in the space, is supportedby the first thin metal film, and is susceptible to corrosion by thecorrosive substance. The measuring terminals are constituted by bothends of the first thin metal film. The first thin metal film and thesecond thin metal film are disposed so that the electric resistancebetween the terminals forms a series circuit including a first parallelcircuit and a second parallel circuit. The first parallel circuitincludes the electric resistance of the second thin metal film beforecorrosion, and the electric resistance of the first thin metal film. Thesecond parallel circuit includes the electric resistance of the secondthin metal film after corrosion, and the electric resistance of thefirst thin metal film.

The present invention provides, in yet another aspect, a method formonitoring a corrosive environment based on the degree of corrosion of athin metal film, where the thin metal film is disposed in at least onechannel that controls entering of a corrosive substance from anatmosphere. The method includes measuring an electric resistance of thethin metal film, where the electric resistance varies depending ongrowth of a corroded region of the thin metal film, where the corrosionin the corroded region is caused by the corrosive substance entering thechannel through an opening of the channel. On the basis of the measuredelectric resistance, the corrosivity of the environment isquantitatively determined.

Advantageous Effects of the Invention

With the present invention, the amount (degree) of corrosion propagatingfrom the opening can be determined accurately with less variation incorrosion amount, and the corrosivity of the environment can bequantitatively determined, where the corrosion amount varies dependingon the flow rate in the atmosphere in the target environment, on localcorrosion (such as corrosion adjacent to a portion where dust and/orsalts are deposited) of the sensor unit, and/or on the thickness of themetal film.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional top view of a corrosive environmentmonitoring device according to Embodiment 1;

FIG. 2 is a cross-sectional side view of the corrosive environmentmonitoring device according to Embodiment 1;

FIG. 3 is a cross-sectional front view of the corrosive environmentmonitoring device according to Embodiment 1;

FIG. 4 is a schematic perspective view of the corrosive environmentmonitoring device according to Embodiment 1;

FIG. 5 is a cross-sectional top view of the corrosive environmentmonitoring device according to Embodiment 1 and illustrates how the thinmetal film is corroded at Time A after exposure;

FIG. 6 is a cross-sectional top view of the corrosive environmentmonitoring device according to Embodiment 1 and illustrates the electricresistance of the thin metal film, as measured at Time A after exposure;

FIG. 7 is a cross-sectional side view of the corrosive environmentmonitoring device according to Embodiment 1 and illustrates how the thinmetal film is corroded at Time A after exposure;

FIG. 8 is a cross-sectional top view of the corrosive environmentmonitoring device according to Embodiment 1 and illustrates how the thinmetal film is corroded at Time B after exposure;

FIG. 9 is a cross-sectional top view of the corrosive environmentmonitoring device according to Embodiment 1 and illustrates the electricresistance of the thin metal film, as measured at Time B after exposure;

FIG. 10 is a cross-sectional side view of the corrosive environmentmonitoring device according to Embodiment 1 and illustrates how the thinmetal film is corroded at Time B after exposure;

FIG. 11 illustrates a table indicating the resistivity and temperaturecoefficient of resistance (TCR) of thin metal films;

FIG. 12 is a schematic cross-sectional top view of a corrosiveenvironment monitoring device according to a comparative examplecorresponding to Embodiment 1;

FIG. 13 is a cross-sectional top view of the corrosive environmentmonitoring device according to the comparative example corresponding toEmbodiment 1 and illustrates the electric resistance of the thin metalfilm, as measured at Time A after exposure;

FIG. 14 is a cross-sectional top view of the corrosive environmentmonitoring device according to the comparative example corresponding toEmbodiment 1 and illustrates the electric resistance of the thin metalfilm, as measured at Time B after exposure;

FIG. 15 is a cross-sectional top view of a corrosive environmentmonitoring device according to Embodiment 2;

FIG. 16 is a cross-sectional side view of the corrosive environmentmonitoring device according to Embodiment 2;

FIG. 17 is a diagram of an equivalent circuit corresponding to theelectric resistance of a thin metal film illustrated in FIG. 19;

FIG. 18 is a diagram of an equivalent circuit corresponding to theelectric resistance of a thin metal film illustrated in FIG. 20;

FIG. 19 is a cross-sectional top view of an area around the thin metalfilms in the initial state;

FIG. 20 is a cross-sectional top view of the area around the thin metalfilms in a state where corrosion proceeds;

FIG. 21 is a cross-sectional side view of the area around the thin metalfilms in the initial state;

FIG. 22 is a cross-sectional side view of the area around the thin metalfilms in the state where corrosion proceeds;

FIG. 23 is a cross-sectional top view of a corrosive environmentmonitoring device according to Embodiment 3;

FIG. 24 is a cross-sectional side view of the corrosive environmentmonitoring device according to Embodiment 3;

FIG. 25 is a cross-sectional top view of the corrosive environmentmonitoring device according to Embodiment 3;

FIG. 26 is a cross-sectional top view of a corrosive environmentmonitoring device according to Embodiment 4;

FIG. 27 is a cross-sectional side view of the corrosive environmentmonitoring device according to Embodiment 4;

FIG. 28 is a cross-sectional top view of a corrosive environmentmonitoring device according to Embodiment 5;

FIG. 29 is a cross-sectional side view of the corrosive environmentmonitoring device according to Embodiment 5;

FIG. 30 exemplarily illustrates the dimensions of a corrosiveenvironment monitoring device according to an embodiment of the presentinvention;

FIG. 31 is a graph illustrating how the electric resistance variesdepending on time, where the electric resistance is the output of thecorrosive environment monitoring device;

FIG. 32 is a graph illustrating how the electric resistance variesdepending on the corrosion thickness of a conventional metal sheet(silver sheet), where the electric resistance is the output of thecorrosive environment monitoring device;

FIG. 33 is a cross-sectional top view of a corrosive environmentmonitoring device in which a thin metal film 11 has a widthapproximately equal to the width of a thin metal film 2;

FIG. 34 is a cross-sectional side view of the corrosive environmentmonitoring device in which the thin metal film 11 has a widthapproximately equal to the width of the thin metal film 2;

FIG. 35 is a cross-sectional top view of a corrosive environmentmonitoring device in which the thin metal film 11 has a larger width ascompared with the width of the thin metal film 2; and

FIG. 36 is a cross-sectional side view of the corrosive environmentmonitoring device in which the thin metal film 11 has a larger width ascompared with the width of the thin metal film 2.

DESCRIPTION OF EMBODIMENTS

Some embodiments of the present invention, which can combine measurementprecision and visibility, will be illustrated with reference to theattached drawings. In these embodiments, there are mainly illustratedembodiments relating to corrosive environment monitoring devices andmethods for measuring the degree of corrosion by a corrosive substancepresent in an environment where electric/electronic equipment is placed.

Embodiment 1

FIGS. 1 to 4 illustrate an exemplary configuration of the corrosiveenvironment monitoring device according to Embodiment 1. FIGS. 1, 2, 3,and 4 are a cross-sectional top view, a cross-sectional side view, across-sectional front view, and an external perspective view,respectively, of the corrosive environment monitoring device.

The corrosive environment monitoring device 1 houses components in ahousing 30, as illustrated in the external perspective view of FIG. 4.The housing 30 has an opening 5 in one side thereof and includes space4. The space 4 is formed by sealing another side opposite to the opening5. The space serves as a channel for a corrosive substance. Atransparent insulating substrate 3 constitutes the underside (in thefigure) of the housing 30 and bears principal components thereon. Thehousing 30 bears, on side faces near the opening 5, terminals 8 a and 8b which extract the output from a sensor disposed inside the housing 30.

FIG. 2 is a cross-sectional side view, when seen from the X-direction inFIG. 4, of the corrosive environment monitoring device 1 illustrated inFIG. 4. On the transparent insulating substrate 3, a thin metal film 2is supported by, and secured to, a thin metal film 11. The thin metalfilm 2 serves as a measuring member. The thin metal film 11 serves as asupporting member. The thin metal film 11 herein is a thin metal filmthat resists (tends to resist) corrosion in a target environment; andthe thin metal film 2 is a thin metal film that is corroded (issusceptible to corrosion) in the target environment. The thin metal film2 is disposed so as to face space 4, where the thin metal film 2 servesas a measuring member, and the space 4 serves as a channel for acorrosive substance. The thin metal film 2 has a slit 20 in a centralpart thereof.

FIG. 3 is a cross-sectional front view, when seen from the Y directionin FIG. 4, of the corrosive environment monitoring device 1 of FIG. 4.FIG. 3 demonstrates that the housing has the opening 5 in one sidethereof, and has another side opposite to the opening 5 being sealed toform the space 4 inside the housing, and that the space is used as achannel for the corrosive substance 6. FIG. 3 also demonstrates that thethin metal film 2 is affected by the corrosive substance, because thethin metal film 2 is disposed so as to be exposed to the space 4 whichserves as a channel for the corrosive substance 6.

FIG. 1 is a cross-sectional top view, when seen from the Z direction inFIG. 4, of the corrosive environment monitoring device 1 of FIG. 4. Withreference to FIG. 1, the measuring member thin metal film 2 issurrounded by, supported by, and secured to, the supporting member thinmetal film 11; and the thin metal film 2 extends toward the depthdirection of the space 4. The slit 20 divides the measuring member thinmetal film 2 to a portion adjacent to the deepmost portion (bottom),where the slit 20 is disposed at the central part of the bottom. Withthis configuration, the electric resistance between the terminals 8 aand 8 b is defined (determined) by the electric resistances of themeasuring member thin metal film 2 and of the supporting member thinmetal film 11. In this case, the electric resistance of the measuringmember thin metal film 2 is affected by corrosion by the corrosivesubstance 6 and is variable.

The corrosive environment monitoring device 1 according to Embodiment 1as exemplarily illustrated in FIGS. 1 to 4 includes a sensor unit. Thesensor unit is disposed as part of a wall in the channel 4 having theopening 5 and includes the thin metal film 2 which is disposed on theinsulating substrate 3. The thin metal films herein includes the thinmetal film 2 and the thin metal film 11, where the thin metal film 2 isexposed to the channel 4, and the thin metal film 11 is electricallycoupled to the thin metal film 2. The thin metal film 2 has the slit 20approximately parallel to the diffusion direction of the corrosivesubstance 6. At both ends of the thin metal film 2, the terminals 8 aand 8 b are disposed to measure an electric resistance. The housing 30itself may be transparent. Such a configuration that at least theinsulating substrate 3 is transparent enables visual observation of howcorrosion of the thin metal film 2 proceeds, from the Z direction inFIG. 2.

Non-limiting examples of a material to constitute the measuring memberthin metal film 2 include aluminum, iron, zinc, and other metalmaterials, in which an elementary metal and its corrosion product differin electric resistance from each other; in addition to copper andsilver, which have been used for corrosion monitoring of an environmentin which electric/electronic equipment is placed. Non-limiting examplesof a material to constitute the supporting member thin metal film 11include such materials as to be approximately uncorroded (resistcorrosion) in the target environment, such as titanium, chromium, gold,palladium, and silver-palladium alloys.

In FIG. 2, the thin metal film 2 and the thin metal film 11 are incontact with each other at side faces of the two thin films. It is alsoacceptable that the two thin films overlap or overlie each other. Inthis case, either one of the thin metal film 2 and the thin metal film11 may be located above the other, with respect to the insulatingsubstrate 3.

When the corrosive environment monitoring device 1 having theconfiguration according to Embodiment 1 is exposed to an environment,the corrosive substance 6 present in the environment enters the channel4 through the opening 5 and corrodes the thin metal film 2, asillustrated in FIG. 3. The channel 4 has the function of controlling therate of corrosion of the thin metal film 2, where the corrosion rate issensing of corrosion by the corrosive substance 6 present in theenvironment. The channel 4 has one opening 5 (left side of the channelin FIG. 1), where the channel has no opening in the right side and isblocked from the ambient environment. This impedes the flow of thecorrosive substance 6 in the ambient atmosphere, if moves toward theopening 5, from entering the channel 4 through the opening 5. In thisview, in corrosive environment monitoring according to the backgroundart, the sensor unit is in direct contact with the flow of the ambientatmosphere, and the corrosion of the sensing metal is promoted when theambient atmosphere flows fast over the surface of the metal.

In the present invention, the corrosive substance 6 adjacent to theopening 5 of the channel 4 enters, by diffusion alone, in the directionof diffusion 7 of the corrosive substance. This allows the corrosiveenvironment monitoring device 1 to measure the degree of corrosion bythe corrosive substance, without being affected by the flow of theambient atmosphere. In the present invention, the corrosion proceeds ina given (constant) direction, and this reduces the measurementvariation. As used herein, the term “corrosive substance” refers to notonly so-called corrosive substances, but also airborne sea salt anddust. A so-called corrosive substance will be described below as arepresentative example of the “corrosive substance”.

Next, a method for quantitatively determining the corrosive substance 6by the corrosive environment monitoring device 1 will be described,where the corrosive substance 6 to be quantitatively determined diffusesinto the channel 4 and is present adjacent to the opening 5.

In Embodiment 1 illustrated in FIGS. 1 to 4, the direction of thediffusion 7 of the corrosive substance 6 is limited to the directionfrom the left side in FIGS. 1 and 3, so as to control corrosion in thecorrosive environment monitoring device 1. With approaching the opening5, the concentration flux of the corrosive substance increases, and thethin metal film 2 is more corroded in the left side approaching theopening 5. This behavior is described in “Estimation of Corrosion Rateof Silver Exposed to Sulfur Vapor” in Journal “Zairyo-to-Kankyo(Corrosion Engineering)”, vol. 56, pp. 265-271(2007). In this article,it is experimentally and analytically determined that the corrosion ratedecreases with an increasing distance from a source of such corrosivesubstance, where the experiments are performed using metal sheets. Usingthis technique, the corrosion behavior of the corrosive environmentmonitoring device 1 can be analyzed.

How the thin metal film is corroded in the corrosive environmentmonitoring device 1 after exposure will be described with reference toFIGS. 5 to 10. FIGS. 5 to 7 illustrate how the thin metal film iscorroded at a certain time (Time A) after exposure, and FIGS. 8 to 10illustrate how the corrosion of the thin metal film proceeds at a latertime (Time B). FIGS. 5, 7, 8, and 10 illustrate corroded regions, andFIGS. 6 and 9 illustrate the electric resistance corresponding to thecorrosion states illustrated respectively in FIGS. 5 and 8.

As illustrated in these figures, the thin metal film 2 used in thepresent invention is corroded no more in a region where the thin metalfilm is corroded to a corrosion thickness equal to the thin filmthickness (region where the corrosion of the thin metal film reaches theinterface with the substrate). The corrosive substance 6 present in theenvironment keeps on diffusing from the left side near to the opening 5and further corrodes the thin metal film in the right side.

As illustrated in FIG. 5 and FIG. 8 in contrast to each other, acorroded region 9 of the thin metal film 2 has a length La at Time A,but extends further to have a length Lb at Time B, where, in thecorroded region 9, the thin metal film 2 is corroded entirely in thethickness direction. Thus, the corroded region 9, in which the thinmetal film 2 is corroded entirely in the thickness direction, is formedin the thin metal film 2 which is exposed to the channel 4, and the thinmetal film 11 alone remains, because the thin metal film 11 is notcorroded in the target environment.

As illustrated in FIG. 6 and FIG. 8 in contrast to each other, theelectric resistance between the terminals 8 a and 8 b is a sum of theelectric resistance Rm of the thin metal film 2 and the electricresistance Ra or Rb of a region where the thin metal film 11 remainsalone, where the thin metal film 11 is not corroded in the targetenvironment. Thus, the electric resistance equals 2Ra+Rm at Time A, andequals 2Rb+Rm at Time B, where Rm<<Ra, Rm<<Rb, and Rm at Time A isapproximately equal to Rm at Time B.

As illustrated in FIG. 7 and FIG. 10 in contrast to each other, the thinmetal film 2 is partially corroded at the surface of the thin metal filmexposed to the channel, in addition to the corroded region 9, where thethin metal film is corroded entirely in the thickness direction. For theconvenience of description, the partial corrosion at the surface of thethin metal film exposed to the channel is not considered herein.

In the corrosive environment monitoring device 1 according to Embodiment1, the sensor unit includes the thin metal film 2 and the thin metalfilm 11 in combination. The embodiment, however, does not merely selectthe thin metal film 2 as a measuring member, and the thin metal film 11as a supporting member, but selects or determines these components inconsideration of the resistivity given in FIG. 11, and of thetemperature.

FIG. 11 illustrates a table indicating the measured values andliterature values of the resistivity, and measured values and literaturevalues of the temperature coefficient of resistance (TCR), of silver Agand silver sulfide Ag₂S as typically examples of the measuring memberthin metal film 2; and of chromium Cr and titanium Ti as typicalexamples of the supporting member thin metal film 11. The measuredvalues demonstrate that silver Ag as the thin metal film 2 has asufficiently lower measured resistivity as compared with the measuredresistivities of chromium Cr and titanium Ti each as the thin metal film11; but, when silver Ag is corroded into silver sulfide Ag₂S, silversulfide has a sufficiently higher measured resistivity as compared withthe measured resistivities of chromium Cr and titanium Ti. The measuredvalues also demonstrate that silver Ag as the thin metal film 2 has ameasured TCR sufficiently higher as compared with the measured TCRs ofchromium Cr and titanium Ti each as the thin metal film 11; and thatthis relationship does not approximately change even when silver Ag iscorroded into silver sulfide Ag₂S.

In the corrosive environment monitoring device 1 according to Embodiment1, the electric resistance of the region in which the thin metal film 11alone remains as a result of corrosion is measured, where the thin metalfilm 11 serves as a sensor unit and is not corroded by corrosion in thetarget environment. Thus, the sensor can have higher sensitivity bymaking the thin metal film 11 from a material having a high electricresistivity. For example, in the examples in FIG. 11, titanium Ti(electric resistivity: 4.27E-7 Ωm) has an electric resistivity 25 foldsthe electric resistivity of silver Ag (electric resistivity: 1.59E-8 Ωm)as the thin metal film in the sensor unit. When the titanium Ti film isdesigned to have a thickness one tenth the thickness of the silver Agfilm (1 μm herein), the sensitivity increases by 250 times.

In Embodiment 1, the thin metal film 2 has the slit 20 extendingapproximately parallel with the direction of the diffusion 7 of thecorrosive substance 6. Advantageous effects of this configuration willbe described below. The description will be made in contrast to acorrosive environment monitoring device 1 without slit 20, asillustrated in FIG. 12. The device in FIG. 12 has configurationsapproximately equal to those of the device in FIG. 1, except for nothaving the slit 20. FIG. 13 illustrates the electric resistance in acorrosion state at a certain time (Time A) after exposure, as with FIG.6. FIG. 14 illustrates the electric resistance in a corrosion state at alater time (Time B) where the corrosion proceeds still more, as withFIG. 9.

As illustrated in FIGS. 13 and 14, the corroded region 9 has a length Laat Time A, whereas the corroded region 9 of the thin metal film 2 growstoward the right side and has a larger length Lb at Time B, in which thethin metal film 2 is corroded entirely in the thickness direction in thecorroded region 9. In the thin metal film 2 exposed to the channel 4,the corroded region 9 in which the thin metal film 2 is corrodedentirely in the thickness direction is formed, and the thin metal film11 alone remains in a conduction portion, where the thin metal film 11is not corroded in the target environment. In this stage, the electricresistance between the terminals 8 a and 8 b is a sum of the electricresistance Rm of the thin metal film 2 and the electric resistance Ra′of the region where only the thin metal film 11 remains, as the thinmetal film 11 is not corroded in the target environment. The electricresistance equals 2Ra′+Rm at Time A, and equals 2Rb′+Rm at Time B.

Herein, the electric resistance Rox of the corroded region 9 in whichthe thin metal film 2 is corroded entirely in the thickness directiondecreases with time with the corroded region 9 grows. However, in theearly stages, the electric resistance Rox is significantly higher thanthe total of 2Ra′ and Rm ((2Ra′+Rm)<<Rox), and the influence of Rox isnegligible. However, when the electric resistance Rox decreases with thegrowth of the corroded region 9 in which the thin metal film 2 iscorroded entirely in the thickness direction and becomes equal to orlower than 2Ra′+Rm (2Ra′+Rm Rox), the electric resistance between theterminals 8 a and 8 b becomes being affected by not only 2Ra′+Rm, butalso by Rox and, in time, is affected (determined) approximately only byRox. Specifically, the electric resistance between the terminals 8 a and8 b in this stage is limited by Rox.

In contrast, when the thin metal film 2 has the slit 20 extendingapproximately parallel with the direction of the diffusion 7 of thecorrosive substance 6 as illustrated in FIG. 1, the electric resistancebetween the terminals 8 a and 8 b is not determined by Rox, but isdetermined by (is in proportional to) the growth of the corroded region9, namely, 2Ra+Rm. This can restrain the device from having decreasingsensitivity with time.

On the basis of these, the configuration of the device according toEmbodiment 1 of the present invention from the viewpoint of the electricresistance of thin metal film can be described as follows. Specifically,this corrosive environment monitoring device includes a housing 30, afirst thin metal film 11, a second thin metal film 2, and measuringterminals 8 a and 8 b. The housing 30 includes space 4 that opens onlyin one side of the housing 30. The first thin metal film 11 is resistantto corrosion by a corrosive substance. The second thin metal film 2 isdisposed in the space 4, is supported by the first thin metal film 11,and is susceptible to corrosion by the corrosive substance. Both ends ofthe first thin metal film 11 constitute the measuring terminals 8 a and8 b. The first thin metal film 11 and the second thin metal film 2 aredisposed so that the electric resistance between the terminals 8 a and 8b forms a series circuit. The series circuit includes a first parallelcircuit and a second parallel circuit. The first parallel circuit isformed by the electric resistance RAg of the second thin metal film 2before the corrosion, and the electric resistance RCr of the first thinmetal film. The second parallel circuit is formed by the electricresistance RAg₂S of the second thin metal film 2 after the corrosion,and the electric resistance RCr of the first thin metal film 11. Thearrangements of thin metal films described in following otherembodiments are intended to meet the above-mentioned conditions inelectric resistance.

Embodiment 2

FIGS. 15 and 16 illustrate another configuration, according toEmbodiment 2, of the corrosive environment monitoring device. FIGS. 15and 16 are a cross-sectional top view and a cross-sectional side view,respectively, of the device. This configuration differs from theconfiguration according to Embodiment 1 in that the terminals 8 a and 8b are disposed not on the side faces adjacent to the opening 5, but at aside opposite to the side having the opening 5. With this configuration,the supporting member thin metal film 11 extends in the channel 4 in adirection from the bottom toward the opening 5 and turns up in the sidehaving the opening 5. The measuring member thin metal film 2 is disposedon both sides of the thin metal film 11 and extends along the thin metalfilm 11. The thin metal film 2 is disposed entirely in the transversedirection (width direction) of the channel 4.

As described above, the corrosive environment monitoring device 1according to Embodiment 2 includes a sensor unit including the thinmetal film 2 disposed on or over the insulating substrate 3. The sensorunit is disposed as part of a wall in the channel 4 which has theopening 5. The thin metal films include the thin metal film 2 and thethin metal film 11, where the thin metal film 2 is exposed to thechannel 4, and the thin metal film 11 is one (rectangular) thin metalfilm disposed under the thin metal film 2 which is exposed to thechannel 4.

Non-limiting examples of a material to constitute the thin metal film 11include materials that resist corrosion in the target environment, suchas titanium, chromium, gold, palladium, and silver-palladium alloys. Thethin metal film 2 has a larger width as compared with the thin metalfilm 11 disposed below the thin metal film 2. Either one of the thinmetal film 2 and the thin metal film 11 may be located above the other,with respect to the insulating substrate 3.

As demonstrated in FIG. 11, the thin metal film 2 (such as a thin silverfilm) has a resistivity of one twentieth the resistivity of the thinmetal film 11 (such as a thin chromium film). On the other hand, thethin silver film has a temperature coefficient of resistance (TCR) of100 folds the temperature coefficient of resistance of the thin chromiumfilm.

FIGS. 17 and 18 are circuit diagrams and illustrate, in the initialstage before exposure and after exposure, respectively, the electricresistance between the terminals 8 a and 8 b of the corrosiveenvironment monitoring device according to Embodiment 2 in the initialstage before exposure. In FIG. 17 which illustrates the circuit in theinitial stage, RAg represents the electric resistance of silver Ag; andRCr represents the electric resistance of chromium Cr. The equivalentcircuit can be indicated as a parallel circuit of these electricresistances. As demonstrated in FIG. 11, RAg is sufficiently lower thanRCr. The current therefore varies depending not on the electricresistance RCr of the thin chromium film, but on the electric resistanceRAg of the thin silver film. The current therefore passes in a largeramperage through the thin silver film (electric resistance RAg).

In contrast to this, the equivalent circuit in FIG. 18 which illustratesthe state where corrosion proceeds, can be considered as two dividedportions. Specifically, a circuit in an uncorroded portion is the sameas with the equivalent circuit in FIG. 17; but a circuit in a corrodedportion can be indicated as a parallel circuit including the electricresistance RAg₂S of a corroded product of silver (silver sulfide) andthe electric resistance RCr of chromium. The circuit as a wholeconstitutes a series circuit of two closed parallel circuits. In thiscase, the current varies depending on the electric resistance RCr of thethin chromium film 11 in a corroded region 9 (Ag₂S) where the thinsilver film is corroded; whereas the current varies depending on theelectric resistance RAg of the thin silver film in the uncorrodedregion. Specifically, in this case, the current flows in a largeramperage from the chromium thin film (electric resistance RCr) side tothe thin silver film (electric resistance RAg) side.

Since the corrosive environment monitoring device 1 is placed even in anenvironment in which the temperature varies, materials to constitute thesensor unit are preferably selected from those having low temperaturecoefficients of resistance (TCRs). As described above, the thin chromiumfilm 11 has a low temperature coefficient of resistance, but the thinsilver film 2 has a temperature coefficient of resistance 100-folds thetemperature coefficient of resistance of the thin chromium film.Accordingly, the structure to be employed herein is preferably such astructure as to minimize the variation in electric resistance of thethin silver film.

FIGS. 19 and 20 are cross-sectional top views, in the initial stage andin the stage where corrosion proceeds, respectively, of an area aroundthe thin metal films. According to Embodiment 2, the thin silver filmhas a larger width (W_(Ag)) as compared with the width (W_(Cr)) of thethin chromium film, as illustrated in FIGS. 19 and 20. Advantageously,control of the electric resistance of the thin silver film 2 to be lowerallows the electric resistance between the terminals 8 a and 8 b to lessvary depending on temperature change. In addition, when a transparentsubstrate is employed as the insulating substrate 3, the above-mentionedconfiguration enables estimation of the degree of corrosion by visualobservation of the length of the corroded region, as observed from thetransparent substrate. FIGS. 21 and 22 are cross-sectional side views ofthe areas around the thin metal films, respectively, in FIGS. 19 and 20.

In Embodiment 2, the periphery of the thin metal film 2 is not incontact with the internal surface of the channel 4. This is because, ifthe periphery of the thin metal film 2 and the internal surface of thechannel 4 are in contact with, or overlap, each other due typically toproduction variation, the corroded region 9 includes an uncorrodedregion, and this causes measured values of electric resistance to vary,where the thin metal film in the corroded region 9 will be corrodedentirely in the thickness direction under the other conditions (undernormal conditions).

Embodiment 3

FIGS. 23 and 24 illustrate another configuration, according toEmbodiment 3, of the corrosive environment monitoring device. FIGS. 23and 24 are a cross-sectional top view and a cross-sectional side view,respectively, of the device. The configuration according to Embodiment 3differs from the configuration according to Embodiment 2 in that themeasuring member thin metal film 2 has a smaller width as compared withthe width of the opening 5 of the channel, in a sensor unit whichincludes the thin metal film 2 and the thin metal film 11 and which isdisposed on (or over) the insulating substrate 3, as illustrated in FIG.23. By the analysis described in “Estimation of Corrosion Rate of SilverExposed to Sulfur Vapor” in Journal “Zairyo-to-Kankyo (CorrosionEngineering)”, vol. 56, pp. 265-271(2007), a sensor unit having a ratioof the width of the thin metal film to the width of the channel of 3:5has a sensitivity about 2.5 folds the sensitivity of a sensor unithaving a ratio of the width of the thin metal film to the width of thechannel of 1:1. The structure (configuration) according to Embodiment 3allows corrosion to proceed faster, and is advantageous for rapidcorrosive environment monitoring which requires high-sensitivitymeasurement.

FIG. 25 illustrates a modification of the corrosive environmentmonitoring device according to Embodiment 3. In this modification, theelectric resistance of the corrosive environment monitoring device 1 ismeasured by a four-terminal measurement method using terminals 8 a 1, 8a 2, 8 b 1, and 8 b 2. This can eliminate or minimize the influence ofconductor resistance.

Embodiment 4

FIGS. 26 and 27 illustrate another configuration, according toEmbodiment 4, of the corrosive environment monitoring device. FIGS. 26and 27 are a cross-sectional top view and a cross-sectional side view,respectively, of the device.

In Embodiment 4, the thin metal film 2 and the thin metal film 11 haveonly to partially overlap each other and to have conduction betweenthem. The thin metal film 2 and the thin metal film 11 may be aligned onone side in the housing. The alignment of the two thin metal films onone side enables easier observation from the transparent substrate 3side to measure the length of the corroded region and enables preciseestimation of degree of corrosion.

Embodiment 5

According to another embodiment (Embodiment 5), a thin metal film 2 anda thin metal film 11 as in Embodiment 4 may be disposed in the channel,as illustrated in FIGS. 28 and 29. FIGS. 28 and 29 are a cross-sectionaltop view and a cross-sectional side view, respectively, of the deviceaccording to Embodiment 5.

The thin metal film 11 is made from a material that is less corroded(resists corrosion) by the environment. The arrangement of the thinmetal film 2 and the thin metal film 11 in the channel actually providesa smaller-sized structure for corrosive environment monitoring.

Next, experimental data and analytical data of a corrosive environmentmonitoring device having a configuration in common among the embodimentsaccording to the present invention will be described. However, thecorrosive environment monitoring device described herein basically hasthe configuration according to Embodiment 4 illustrated in FIGS. 26 and27, further employs the four-terminal structure illustrated in FIG. 25,and has dimensions illustrated in FIG. 30.

The corrosive environment monitoring device having the dimensionsillustrated in FIG. 30 employed, as a sensor unit, a thin silver filmhaving a thickness of 100 nm as the thin metal film 2, and a thinchromium film having a thickness of 100 nm as the thin metal film 11 inthe channel. The components have dimensions as given in the figure. Theenvironment to which the device is exposed is an accelerated environmentderived from the actual environment and contains 1.0 ppm of NO₂, 1.0 ppmof SO₂, and 0.5 ppm of H₂S, at a temperature of 35° C. and humidity of75%.

FIG. 31 illustrates how the electric resistance varies depending on thetime, where the electric resistance is an output of the corrosiveenvironment monitoring device. The analytical data demonstrates that theexperimental data agree with the analytical data highly precisely on thechange with time in electric resistance of the sensor.

FIG. 32 illustrates how the electric resistance of the corrosiveenvironment monitoring device varies depending on the corrosionthickness of a conventional metal sheet (silver sheet), where theelectric resistance is an output of the corrosive environment monitoringdevice. The analytical data demonstrates that the experimental dataagree with the analytical data highly precisely.

Assume that the electric resistance is measured while exposing thecorrosive environment monitoring device to a target environment, and thecorrosion thickness of a metal test specimen is calculated in the abovemanner. This enables classification of the corrosivity of the ambientatmosphere on the basis of the calculated corrosion thickness, inaccordance with any of IEC 654-4 standard, ISO 11844-1 standard, ISO9223 standard, and ISA 71.04 standard.

When a transparent substrate is used as the insulating substrate 3, thecorroded region 9, where the thin metal film is corroded entirely in thethickness direction, can be visually observed, and the lifetime of thesensor can be checked in situ. With an increasing corrosive substanceconcentration in the environment to be assessed, the metal corrosionrate increases, and the electric resistance also increases.

The corrosive environment monitoring device according to the presentinvention may include a measurement system in isolation. The corrosiveenvironment monitoring device may also have a configuration in which thedevice is mounted typically on a printed circuit board and uses ameasurement system which has been previously configured on or in theprinted circuit board. The corrosive environment monitoring device mayalso be mounted on a printed circuit board to enable self-diagnosis ofthe resulting electronic equipment.

The sensitivity of the corrosive environment monitoring device 1according to any of embodiments of the present invention is determinedby the ratio in electric resistance among the thin metal film 2, thecorrosion product 9 of the thin metal film 2, and the thin metal film11. In electric resistance, a preferred relationship among them is: thinmetal film 2<thin metal film 11<<corrosion product 9 of thin metal film2.

These electric resistances are determined by the thickness, width, andlength, as well as electric resistivity, of the thin films. Assume thatthe thin metal film 11 has an electric resistivity significantly higheras compared with the electric resistivity of the thin metal film 2, andthis impedes the measurement of the electric resistance change in thecorrosive environment monitoring device. In this case, it is preferredto allow the thin metal film 11 to have a larger width as compared withthe width of the thin metal film 2. This can reduce the electricresistance change in the corrosive environment monitoring device.

Specifically, it is advantageous to allow the thin metal film 11 to havea width approximately equal to the width of the thin metal film 2, asillustrated in FIGS. 33 and 34. For further reducing the change inelectric resistance of the corrosive environment monitoring device, itis more advantageous to allow the thin metal film 11 to have a largerwidth as compared with the width of the thin metal film 2, asillustrated in FIGS. 35 and 36. FIGS. 33 and 35 are cross-sectional topviews, and FIGS. 34 and 36 are cross-sectional side views.

As described above, the configurations according to the presentinvention do not require a suction pump in a gas introducing unit oranother large-sized structure capable of performing measurement in anenvironment at a constant flow rate. The configurations thereby lessconsume power and enable simple measurement. The configurationsaccording to the present invention have an opening in part of thechannel and employs a thin metal film covered by (exposed to) thechannel. This enables accurate determination of the amount (degree) ofcorrosion of the thin metal film, where the corrosion proceeds from theopening, and can restrain the variation in corrosion amount (degree),where the corrosion amount (degree) varies depending on the flow rate inthe atmosphere in the target environment; on local corrosion (such ascorrosion adjacent to a portion where dust and/or salts are deposited)of the sensor unit; and/or on the thickness of the metal film. When atleast the substrate is transparent, the corrosion amount (degree) can beeasily visually observed.

Many embodiments have been described above, and all of them have theconfiguration as follows in common, and on the basis of the commonconcept (configuration), some modifications, variations, and equivalentarrangements have been made. Specifically, the corrosive environmentmonitoring device having the configuration includes a housing, a firstthin metal film, a second thin metal film, and terminals. The housinghas an opening in one side thereof. The other sides of the housing thanthe one side are sealed to form space inside the housing. The first thinmetal film extends in a direction from the bottom of the space towardthe opening. The first thin metal film is resistant to corrosion by acorrosive substance and serves as a supporting member. The second thinmetal film extends in the space in a direction from the bottom of thespace toward the opening and is supported by the first thin metal film.The second thin metal film is susceptible to corrosion by the corrosivesubstance and serves as a measuring member. The terminals are disposedat both ends of the first thin metal film, where an external voltage isto be applied to the terminals. The first thin metal film includes onefirst thin metal film extending in a direction from the bottom of thespace toward the opening. The second thin metal film is disposed on oneor both sides of the one first thin metal film and extends in the spacein a direction from the bottom toward the opening.

LIST OF REFERENCE SIGNS

1: corrosive environment monitoring device,

2: thin metal film,

3: insulating substrate,

4: space,

5: opening,

6: corrosive substance,

8 a, 8 b: terminal,

9: corroded region,

11: thin metal film,

30: housing.

1. A corrosive environment monitoring device comprising: a housinghaving an opening in one side thereof, other sides of the housing thanthe one side being sealed to form space inside the housing; a first thinmetal film extending in a direction from a bottom of the space towardthe opening, the first thin metal film being resistant to corrosion by acorrosive substance and serving as a supporting member; a second thinmetal film extending in the space in a direction from the bottom of thespace toward the opening and being supported by the first thin-filmmetal, the second thin metal film being susceptible to corrosion by thecorrosive substance and serving as a measuring member; and terminalsbeing disposed at both ends of the first thin metal film, where anexternal voltage is to be applied to the terminals, the first thin metalfilm including one first thin metal film extending in a direction fromthe bottom of the space toward the opening, the second thin metal filmbeing disposed on one or both sides of the one first thin metal film andextending in the space in a direction from the bottom toward theopening.
 2. The corrosive environment monitoring device according toclaim 1, wherein the first thin metal film is disposed in a U shapealong the space so as to support the first thin metal film by threesides of the U-shaped first thin metal film, where the three sidesexclude a side facing the opening, and wherein the second thin metalfilm has a slit extending from the center of the opening toward thebottom of the space.
 3. The corrosive environment monitoring deviceaccording to claim 1, wherein the first thin metal film extends from thecenter of the opening toward the bottom of the space, and wherein thesecond thin metal film is disposed on one or both sides of the firstthin metal film and extends in the space from the bottom toward theopening.
 4. The corrosive environment monitoring device according toclaim 3, wherein the second thin metal film extends in the space to thebottom of the space.
 5. The corrosive environment monitoring deviceaccording to claim 3, wherein the second thin metal film is disposed ina part of the space.
 6. The corrosive environment monitoring deviceaccording to claim 1, wherein the terminals are a pair of terminalsdisposed on an opposite side of the space to the opening, wherein thefirst thin metal film is a U-shaped supporting member disposed so as tocouple the pair of terminals to each other, the U-shaped supportingmember including: a first portion extending in a direction from one ofthe pair of terminals toward the opening; a second portion extending ina direction from the opening to the other of the pair of terminals; anda third portion disposed between the first portion and the secondportion at the side having the opening, and wherein at least one of thefirst and second portions of the U-shaped supporting member is presentin the space and supports the first thin metal film.
 7. The corrosiveenvironment monitoring device according to claim 1, wherein the firstthin metal film is held by a substrate portion of the housing, andwherein at least the substrate portion of the housing is opticallytransparent.
 8. The corrosive environment monitoring device according toclaim 1, wherein the terminals disposed at both ends of the first thinmetal film, to which an external voltage is to be applied, have afour-terminal structure.
 9. The corrosive environment monitoring deviceaccording to claim 1, wherein the second thin metal film beforecorrosion has a sufficiently lower electric resistance as compared withthe electric resistance of the first thin metal film, and wherein thefirst thin metal film after corrosion has a sufficiently higher electricresistance as compared with the electric resistance of the first thinmetal film.
 10. The corrosive environment monitoring device according toclaim 1, wherein the device measures an electric resistance of thesecond thin metal film, where the electric resistance varies dependingon growth of a corroded region of the second thin metal film, and wherethe growth of the corroded region is caused by the corrosive substanceentering the space through the opening.
 11. The corrosive environmentmonitoring device according to claim 10, wherein the corroded region ofthe second thin metal film grows in a direction along which thecorrosive substance entering through the opening diffuses, and whereinthe device measures the electric resistance of the second thin metalfilm, where the electric resistance increases with the growth of thecorroded region.
 12. The corrosive environment monitoring deviceaccording to claim 11, wherein the electric resistance is measured basedon a sum of the electric resistance of the first thin metal film and theelectric resistance of the second thin metal film.
 13. The corrosiveenvironment monitoring device according to claim 12, wherein theelectric resistance of the second thin metal film increases with thegrowth of the corroded region.
 14. The corrosive environment monitoringdevice according to claim 1, wherein the first thin metal film is madefrom a material containing at least one selected from the groupconsisting of titanium; chromium; gold; palladium; and silver-palladiumalloys.
 15. The corrosive environment monitoring device according toclaim 1, wherein the second thin metal film is made from a materialcontaining at least one selected from the group consisting of copper;silver; aluminum; iron; and zinc.
 16. A corrosive environment monitoringdevice comprising: a housing including space that opens only in one sideof the housing; a first thin metal film being resistant to corrosion bya corrosive sub stance; a second thin metal film being disposed in thespace, being supported by the first thin metal film, and beingsusceptible to corrosion by the corrosive substance; and measuringterminals being constituted by both ends of the first thin metal film,wherein the first thin metal film and the second thin metal film aredisposed so that an electric resistance between the terminals forms aseries circuit which includes: a first parallel circuit being formed ofan electric resistance of the second thin metal film before thecorrosion, and an electric resistance of the first thin metal film; anda second parallel circuit being formed of an electric resistance of thesecond thin metal film after the corrosion, and the electric resistanceof the first thin metal film.
 17. The corrosive environment monitoringdevice according to claim 16, wherein the second thin metal film beforethe corrosion has a sufficiently lower electric resistance as comparedwith the electric resistance of the first thin metal film, and whereinthe first thin metal film after corrosion has a sufficiently higherelectric resistance as compared with the electric resistance of thefirst thin metal film.
 18. The corrosive environment monitoring deviceaccording to claim 16, wherein the second thin metal film has asufficiently higher temperature coefficient of resistance as comparedwith the temperature coefficient of resistance of the first thin metalfilm.
 19. A method for monitoring a corrosive environment based on adegree of corrosion of a thin metal film, the thin metal film beingdisposed in at least one channel that controls entering of a corrosivesubstance from an atmosphere, the method comprising the steps of:measuring an electric resistance of the thin metal film, the electricresistance varying depending on growth of a corroded region of the thinmetal film, the corrosion in the corroded region being caused by thecorrosive substance entering the channel through an opening of thechannel; and quantitatively determining a corrosivity of the environmentbased on the measured electric resistance.